Jets hit towers in most vulnerable spots / Killers appear to have known where to strike

Plumes of smoke poured from the World Trade Center buildings in New York after planes crashed into the upper floors of both towers. The Empire State building is seen in the foreground. Associated Press photo by Patrick Sison less

Plumes of smoke poured from the World Trade Center buildings in New York after planes crashed into the upper floors of both towers. The Empire State building is seen in the foreground. ... more

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Plumes of smoke poured from the World Trade Center buildings in New York after planes crashed into the upper floors of both towers. The Empire State building is seen in the foreground. Associated Press photo by Patrick Sison less

Plumes of smoke poured from the World Trade Center buildings in New York after planes crashed into the upper floors of both towers. The Empire State building is seen in the foreground. ... more

Jets hit towers in most vulnerable spots / Killers appear to have known where to strike

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The two planes that slammed into the twin towers of New York's World Trade Center triggered a "progressive collapse" of the 110-story structures in a monumental failure of all the steel columns and beams that had held both structures upright, engineers said yesterday.

The planes that burst into flames as they hit -- and the explosions of their fuel that followed -- created an inferno that weakened and then destroyed the supporting steel elements of the 1,360-foot-high twin towers, the engineers said.

The aim of the pilots into the upper floors of the buildings was so precise,

one engineer speculated, that the terrorists who planned the attack must have known precisely how to destroy the massive steel-and-concrete landmarks.

"This is the worst possible part of the building that could possibly be hit.

The lower the planes could hit the buildings, the more damage they could cause. But the planes had to fly at altitude of about 60 stories to skirt nearby buildings. The first tower was hit at about the 80th story. The second tower was hit at about the 60th story.

The towers were built to withstand the enormous stresses of hurricane-force winds and to survive the heat of ordinary fires. And one of the engineers who did the structural design in the 1960s even claimed after the Trade Center bombing in 1993 that each tower had been built to withstand the impact of a fully loaded, fully fueled Boeing 707, then the heaviest aircraft flying.

But no engineer could have prepared for this, experts said.

The crash of the planes themselves must have had little, if any, impact on the internal structure of the towers at first, Astaneh explained, as the planes' weight alone -- even at a few hundred thousand pounds each -- would not have ruptured the girders.

But the thousands of gallons of burning fuel and the instant explosions produced temperatures of more than 2,000 degrees, Astaneh said. That was more than enough to destroy the insulation around the steel beams and columns and weaken the steel members until they became "soft and mushy," he said.

Those structural columns and beams, however strong originally, could not possibly have supported the undamaged upper floors of the two towers above the explosions. Suddenly, "as the columns collapsed, those whole upper stories of the buildings dropped like dead bodies," Astaneh said.

"That impact was too much, and no building could possibly withstand such weight, so floor after floor came down in what we call progressive collapse," he said.

In 1993, a terrorist car bomb badly damaged the lower floors of the One World Trade Center tower, killing six and injuring more than 1,000 people. But that bomb could not have collapsed the entire building, Astaneh said, because the columns and beams on the lower floors were 3 to 4 feet thick -- "so huge, so thick that collapse was impossible."

If yesterday's attack by aircraft had hit only a few of the top floors of the towers, they might have been destroyed, but the weight of their concrete and their thinner beams and girders would not have caused more collapse below, and the rest of the buildings would have remained intact, Astaneh said.

"So, this attack was like shooting a person in the heart, the most vulnerable part of the body," Astaneh said as he recalled his horror at what he saw. "Even I couldn't believe that gravity was pulling those buildings down. "

Richard Behr, an architect and engineer at Pennsylvania State University in State College, Pa., specializes in designing "envelope systems" that help make large buildings resist catastrophic damage.

The impact of the two planes, Behr said in a telephone interview, "easily pierced the exterior of the towers, and as they exploded in flames, they must have touched off a fire that quickly began melting the internal structural elements.

"It looks like somebody really knew what he was doing if he planned to send those planes right at the target areas of the buildings they seem to have chosen," he said.

And that means that the terrorists must have known how to plan the most effective possible attack against the World Trade Center, Behr said.

"It's a frightening situation because there's no realistic way to design buildings against that kind of attack -- no way without exorbitant costs. We'd have to build all large buildings that might be vulnerable to terrorism like the containment structures of nuclear power plants, and that's absolutely unthinkable," he said.

"It was clear from the way the blast and fire weakened the structural elements of the floors where the planes hit, that progressive collapse of the two towers was inevitable," Krauthammer said.

"I believe that any tall building which has symbolic value for Americans, or whose mission is critical, could be threatened in the same way," he said. "And people should not be surprised if it happens again."